Links should change colors when visited to assist the user in navigation. This may be common knowledge to you. Nevertheless, from time to time I run into graphical designers that are not aware of this web convention and why it should be so.
Links That Don’t Change Color When Visited
The oldest usability guideline for any type of navigational design is to help users understand where they’ve been, where they are, and where they can go — their online past, present, and future. The three are somewhat interrelated: A good grasp of past navigation helps you understand your current location, since it’s the culmination of your journey. Knowing your past and present locations in turn makes it easier to decide where to go next.
On the web, links are a key factor in this navigation process. Users can stop using links that proved fruitless in the past. Conversely, they might revisit links they found helpful in the past. Most important, when users know which pages they’ve already visited, they are less likely to unintentionally revisit them.
The Exception: Command-Oriented Functionality
Command-oriented functionality is the exception to this rule. Showing visited areas is unnecessary for applications in which people might want to repeat actions multiple times. When deciding whether to show visited areas, consider if the action takes people to other screens or merely lets them repeat activities on the same screen. If users are going to other areas, especially to get content, they might only need one visit, so showing a visited link is appropriate. However, if people want to repeat an activity on the same screen, showing visited links is unnecessary.